The major merozoite surface protein of Plasmodium species has been shown to be a target of varying degrees of protective immunity against the asexual blood stages in rodent and human malaria. For example, vaccination of mice with purified P230, the major merozoite surface protein of the rodent malaria Plasmodium yoelii, has resulted in reduced parasitemias in comparison to controls upon intravenous challenge with a lethal dose of parasitized erythrocytes (Holder et al. 1981. Nature 294:361). Mice have also been protected against P. yoelii by passive transfer of a monoclonal antibody (Mab) specific for P230 (Majarian et al. 1984. J. lmmunol. 132:3131) and against (rodent malaria) Plasmodium chabaudi adami challenge by passive immunization with a Mab specific for the homologous 250-kDa molecule of this plasmodium species (Lew et al. 1989. Proc. Natl. Acad. Sci. USA 86:3768). The ability to confer resistance to parasite challenge by passive transfer of antibodies suggests that antibody-mediated mechanisms play an important role in antigen-specific immunity to malaria.
Despite these findings, however, no commercially viable vaccine has been developed against the major merozoite surface antigen of the major human malaria pathogen, Plasmodium falciparum. 
For example, using naturally derived materials, such as the precursor of the major merozoite surface protein (MSP) alone (gp195: 195,000-200,000 Da molecular species; merozoite surface protein-I (MSP- 1)), gp195 mixed with certain of its natural processing fragments, or a natural processing fragment by itself, partial protection against Plasmodium falciparum infection has been achieved by some researchers (Hall et al. 1984. Nature 311:379; Perrin et al. 1984. J. Exp. Med. 160:441; Patarroyo et al. 1987.
Vaccines 87 (Brown, Chanock, Lerner, ed.) Cold Spring Harbor Laboratory Press, CSH, N.Y. 117-124). An effective vaccine against Plasmodium falciparum should not convey merely partial protection, i.e. a partial lowering of parasitemia, since even low parasitemias of this organism cause serious illness. A commercially useful vaccine should substantially eliminate parasitemia. It is generally thought that an acceptable malaria vaccine that would reduce parasitemia to a low level and, consequently, result in a substantial reduction in morbidity and mortality.
In one instance, substantially complete protection using natural materials against Plasmodium falciparum challenge has been achieved in Aotus monkeys, in particular by the use of a mixture of gp195 and some of its natural processing fragments obtained by affinity purification using a Mab, designated Mab 5.2 (Siddiqui et al 1987. Proc. Natl. Acad. Sci. USA 84:3014). In follow up experiments using Mab 5.2 affinity purified, parasite gp195, a correlation was found between protection against infection with Plasmodium falciparum and the ability of serum antibodies to strongly inhibit parasite growth in vitro. In particular, monkeys and rabbits hyperimmunized with Mab 5.2 affinity purified parasite gp195 in complete Freund's adjuvant produced antibodies that inhibited in vitro parasite growth (Hui et a. 1987. Exp. Parasitol. 64:519).
The difficulty in developing an effective naturally derived vaccine, however, has been compounded with the difficulty in developing an effective recombinant or synthetic vaccine. Recombinant or synthetic vaccines are desirable for several reasons. They have the potential to focus immune response(s) on the most effective portion of gp195, an important advantage since there may be decoy determinants in gp195 which prevent the most effective response. Also, more homogeneous preparations are possible using recombinant techniques than in preparations of naturally derived products. In addition, recombinant and synthetic based vaccines avoid the potential contamination of naturally derived gp195 with pathogens from its human source.
Although a number of investigators have designed and tested gp195-based synthetic peptides and recombinant products as vaccine antigens, no strongly protective vaccine has resulted. Thus, synthetic peptides corresponding to various segments of the N-terminal 83 kDa processing fragment of gp195 induced antibodies in rabbits which displayed only a low level of cross reactivity with asexual blood stage parasites (Cheung et al. 1986. Proc. Natl. Acad. Sci. USA 83:8328). One of these synthetic peptides, corresponding to a non-repetitive, conserved sequence, partially protected Saimiri monkeys against Plasmodium falciparum challenge (Cheung et al. 1986). In a vaccination study in Aotus monkeys using an 83 kDa processing fragment-based recombinant polypeptide produced in E. coli there was no significant difference between the course of infection of control animals and animals immunized with the recombinant polypeptide. In addition, very low levels of antibodies cross-reactive with native gp195 by immunofluorescence were induced (Knapp et al. 1988. Behring Inst. Mitt. 82:349). A bacterial recombinant polypeptide based on a fusion of two conserved regions located towards the amino terminus and center of the gp195 molecule induced only low indirect fluorescent antibody (IFA) titers when used to immunize Aotus monkeys (Herrera et al. 1990. Proc. Nati. Acad. Sci. USA 87:4017) and two out of five immunized animals were partially protected.
Holder et al. studied two recombinant polypeptides which corresponded to portions of the 42 kDa C-terminal processing fragment of gp195 (p42) fused to trp E and beta-galactosidase carrier sequences, respectively (Holder et al. 1988. Parasite. Immunol. 10:607). While immunized animals produced high antibody titers against the carrier portion of the recombinant polypeptides, much lower titers were detected against the gp195 antigen. Some of the Aotus monkeys immunized with both of these recombinant polypeptides were partially protected against parasite challenge.
Murphy et al. (1990. Parasitology. 2:177-183) attempted to recombinantly produce portions of the p42 antigen of the Wellcome isolate of gp195 in insect host cells. gp195 is believed to exist in at least two allelic forms, of which the Wellcome isolate (“Wellcome allele”) and the MAD isolate (“MAD allele”) are representative (Tanabe et al. 1987. J. Mol. Biol. 195:273). While Murphy et al. reported producing a product which folded in a similar manner to the natural antigen, they did not report obtaining a purified polypeptide but only reported multiply banded antigens speculated to have resulted from post-translational processing or degradation. No follow-up immunogenicity or efficacy studies have been reported using any materials obtained.
A mixture of three synthetic peptides, one peptide from the 83 kDa processing fragment of gp195 and two non-gp195 malaria peptides, partially to completely protected monkeys against parasite challenge; a hybrid synthetic polymer including the sequences of the three synthetic peptides in addition to a circumsporozoite region was reported to provide a delay or suppression of parasitemias (Patarroyo et al. 1987. Nature 328:629; Rodriguez et al. 1990. Am. J. Trop. Med. Hyg. 43:339; Patarroyo et al. 1988. Nature 332:158). Field trials of this hybrid are under way. It is unclear whether any gp195 epitopes in the mixture or hybrid resulted in any protection. In addition there have been two reported studies which were unable to duplicate the prior results obtained using the peptide mixture (Reubush et al. 1990. Am. J. Trop. Med. Hyg. 43:355-366) or the hybrid peptide multimer (Herrera et al. 1991. Abstract in the IV International Congress on Malaria and Babesiosis). Thus, there has been no gp195-based recombinant or synthetic vaccine antigen which has been shown sufficiently effective against Plasmodium falciparum challenge.
Accordingly, it is an object of the invention to provide recombinant or synthetic antigens, compositions comprising these antigens, and methods of use that are effective against Plasmodium falciparum challenge.